I. Why was the decree only for the generation that entered the land?
Baal Hatur Bm.25:5 sites the word (יִנְחָלוּ-inherit) and elaborates that this word is mentioned 3 times in TNH.
Bm.25:5 - according to his inheritance that they shall inherit.
Bm.26:55 - according to the names of their fathers' tribes shall they inherit.
Pr.3:35 - The wise shall inherit honor.
The Tur states in connection to these 3 passages: Regarding that which they shall inherit, shall be according to the names of their father's tribes that they shall inherit, for the land was apportioned to those who went forth from Mizraim and died...
The Gemara in Baba Batra 117a: Rabbi Yonatan says: Eretz Yisrael was divided among those who entered Eretz Yisrael, as it is stated: “Unto these the land shall be divided for an inheritance” (Bm 26:53). But how do I realize the meaning of the verse: “According to the names of the tribes of their fathers they shall inherit” (Bm 26:55)? This teaches that this inheritance is different from all other inheritances in the world, for in all other inheritances in the world, the living inherit from the dead, but here, the dead inherit from the living. {In other words, the portions of land received by those who entered Eretz Yisrael were transferred to their fathers who left Egypt, and then inherited by the current generation, as the Gemara will explain.} 117a
The reason for this nuance in my opinion is for the initial promise to Abraham in (Br.12.7: The LORD appeared to Abram and said, “I will assign this land to your offspring.” And he built an altar there to the LORD who had appeared to him.) Is defining that the offspring of Abraham will inherit the land, the only question is what that "offspring" really mean. This is to some point defined in (Br.21.12: But God said to Abraham, “Do not be distressed over the boy or your slave; whatever Sarah tells you, do as she says, for it is through Isaac that offspring shall be continued for you.)
It is also important that the language of the Torah throughout the Parasha Masei is Bnei Israel (the males), therefore it can be established that it is critical that the males are the ones that inherited.
Therefore you see that the initial inheritance was not a commandment, but HSM holding his promise to Abraham, Itzhak and Yaakov. But once the inheritance is established and the land is conquered and acquired (as needed to qualify for a possession) then there are specific mitzvot from the Torah on how things now need to be handled in cases of inheritance and possession of the land.
II. Why is a female heiress allowed to marry out of the tribe?
There was never a restriction for marriage out of the tribe, only a one time decree set for the initial inheritance of the land. The reason being, the legitimate plea of the daughters of Zelophehad: "Why should our father lose out because he has no sons" (referencing Baba Batra 117a). In addition, the tribe of Menashe also had a legitimate argument, why should they lose the portion of their tribal land to another tribe when it belongs to them (referencing "offspring according to the names of the fathers' tribes".) And if the daughters marry out, they don't even have the chance to "redeem it" so to say. The solution for the daughters of Zelophehad seems to be infringing on the possession of the Tribe. Therefore, HaKadosh Barkhu put a restriction for these daughters to only marry within the tribe. A seemingly fair compromise to both parties.
The concept of redemption can be see in Ruth 4.4: [I thought I should disclose the matter to you and say: Acquire it in the presence of those seated here and in the presence of the elders of my people. If you are willing to redeem it, redeem! But if you will not redeem, tell me, that I may know. For there is no one to redeem but you, and I come after you.” “I am willing to redeem it,” he replied.]
III. The Maharsha (Chidushei Aggadot BB 121A) (Discussing the holiday of the 15th of Av) asks what is the joy of the tribes being able to intermarry, if this could result in lost land.?
(added from comments:) Take a real life example from sefer Ruth:
Elimelech a rich man with much land lost his life due to his own sins. His possessions transfer to his living sons (his guarantors). Mahlon and Chilion died early due to their own sins, and lost all possessions. They however had no sons and no more brothers to perform Yibum. Their "names" are lost forever since they didn't guard this mitzvah properly of having children. Now since all of Am-Yisrael are guarantors for one another, it trickles down the "family" tree.
The possessions now belong to Naomi, and the family of Elimelech now have the status of potential redeemers. Tov refused to redeem, Boaz the last relative acts as the redeemer and states Ruth 4.10: I also acquire the wife of Machlon as my wife to perpetuate the name of the deceased on his inheritance, that the name no be cut off ... Now what would happen if Boaz also refused or as in the case where the entire tribe doesn't redeem? The real loss are to those to whom the original inheritance belonged to.
Therefore it seems to me that life is more important than land and the purpose of inheritance as the word itself implies is to perpetuate the name of the deceased. Indeed this should be something to celebrate, both in not restricting the daughters from marrying whom they want and the inheritance transferring over to a different tribe. Since all of Am Yisrael are guarantors for one another, it is not that the land was lost, but that the name of the deceased was not cut off from his people, which remains on the possessions that he leaves in this world, in this case land.